When a C0 character is present in an OSC terminator (i.e. after the ESC
but before a \ (0x5c) or printable character), vterm executes the
control character and resets the current string fragment. If the C0
character is the final byte in the sequence, the string fragment has a
zero length. However, because the VT parser is still in the "escape"
state, vterm attempts to subtract 1 from the string length (to account
for the escape character). When the string fragment is empty, this
causes an underflow in the unsigned size variable, resulting in a buffer
overflow.
The fix is simple: explicitly check if the string length is non-zero
before subtracting.
Problem: Contents of terminal buffer are not reflown when Nvim is
resized.
Solution: Enable reflow in libvterm by default. Now that libvterm is
vendored, also fix "TUI rapid resize" test failures there.
Note: Neovim's scrollback buffer does not support reflow (yet), so lines
vanishing into the buffer due to a too small window will be restored
without reflow.
Problem: Adding support for modern Nvim features (reflow, OSC 8, full
utf8/emoji support) requires coupling libvterm to Nvim internals
(e.g., utf8proc).
Solution: Vendor libvterm at v0.3.3.
